The present invention is generally related to ultrasound imaging systems, and more particularly, to an ultrasound imaging system and method that employs spatial compounding to reduce speckle in ultrasound imaging.
Ultrasonic imaging has become an important and popular diagnostic tool with a wide range of applications. Particularly, due to its non-invasive and typically non-destructive nature, ultrasound imaging has been used extensively in the medical profession. Modem high-performance ultrasound imaging systems and techniques are commonly used to produce two-dimensional diagnostic images of internal features of an object (e.g., portions of the anatomy of a human patient). A diagnostic ultrasound imaging system generally uses a wide bandwidth transducer to emit and receive ultrasound signals. The ultrasound imaging system forms images of the internal tissues of a human body by electrically exciting an acoustic transducer element or an array of acoustic transducer elements to generate ultrasonic pulses that travel into the body. The ultrasonic pulses produce echoes as they reflect off of body tissues that appear as discontinuities to the propagating ultrasonic pulses. The various echoes return to the transducer and are converted into electrical signals that are amplified and processed to produce an image of the tissues. These ultrasonic imaging systems are of significant importance to the medical field by providing physicians real-time high-resolution images of internal features of a human anatomy without resort to more invasive exploratory observation techniques such as surgery.
Ultrasonic imaging systems employ an acoustic transducer to radiate and receive a plurality of ultrasonic pulses. The acoustic transducer, which radiates the ultrasonic pulses, typically comprises a piezoelectric element or an array of piezoelectric elements. As is known in the art, a piezoelectric element deforms upon application of an electrical signal to produce the transmitted ultrasonic pulses. Similarly, the received echoes cause the piezoelectric element to deform and generate a corresponding receive electrical signal. The acoustic transducer is often packaged in a handheld device that allows an operator substantial freedom to manipulate the transducer over a desired area of interest. The transducer is often connected via a cable to a control device that generates and processes the electrical signals. In turn, the control device may transmit image information to a real-time viewing device, such as a display monitor. In alternative configurations, the image information may also be transmitted to physicians at a remote location and or stored in a recording device to permit viewing of the diagnostic images at a later time.
One fundamental problem in all types of ultrasound imaging is noise from back-scattered signals, which obscures the details of the target image or echo. One type of noise, commonly known as xe2x80x9cspeckle,xe2x80x9d results from constructive and destructive interference, and appears as a random mottle superimposed on the image. Normally, speckle is received from objects having dimensions smaller than the wavelengths generated by the ultrasound energy source, making it impossible to reduce the speckle simply by increasing the resolution of the device. Moreover, speckle originates from objects that are stationary and randomly distributed. Since the speckle has no phase or amplitude variation over time, one cannot suppress the speckle by averaging the image signals over time. In other words, speckle signals are coherent and cannot be reduced by time averaging.
One way to reduce speckle is through a method known as spatial compounding. The idea is to insonify a target image with ultrasonic energy and receive or capture the target image from a number of different vantage points. The multiple received images related to each of the various vantage points are then mathematically combined to reduce the speckle. The success of the method is due to the statistical independence of the speckle patterns when viewed from multiple vantage points, and the fact that the target size is much larger than the speckle causing scatterers. By mathematically combining (e.g., averaging) a plurality of images formed from information gathered from a number of vantage points, the speckle patterns lack correlation, while the target echoes remain correlated and virtually unchanged. As a result of the lack of correlation in the speckle patterns between the various vantage points, the variance in the speckle patterns can be reduced without degrading the target image. The calculations to mathematically combine images formed from different vantage points for reducing speckle are well known.
There are two known methods for generating a spatially compounded ultrasound image. A first method, uses a conventional transducer that is moved to various vantage points with an articulated arm to acquire the necessary images. The transducer location is accurately measured by sensing devices in order to locate each of the images. An example of a compound image scanner using angular sensing devices on an arm assembly is disclosed in U.S. Pat. No. 4,431,007, to Amazeen et al., entitled, xe2x80x9cReferenced Real-Time Ultrasonic Image Display.xe2x80x9d In practice, however, the arm assembly is awkward and inflexible to operate, and the sensing devices add significant complexity and cost to the ultrasonic imaging system. A second technique uses a transducer having an array of transducer elements to generate two or more images at slightly different viewing angles from a fixed transducer location.
Typically, the way to generate multiple images from different directions with a xe2x80x9cfixedxe2x80x9d transducer is to excite different cells or groups of cells of a linear or curved linear array of piezoelectric transducer elements, which are used to generate and receive the ultrasound energy. The vantage point for an ultrasound beam is typically controlled by the physical position of an active aperture used for forming the ultrasound beam. Thus the groups in a fixed transducer must be separated along the array in order to achieve the required spatially separated vantage points.
By way of example, one can separate a linear array of N transducer elements into M sections, each section having N/M contiguous transducer elements and defined by an unique location or vantage point along the array. Each section may be electrically excited one at a time in succession with the resulting ultrasound beam from each of the transducer sections steered so that all M beams are focused at substantially the same region, but from different directions having their origin at the face of the transducer array. Speckle can then be reduced by combining the M ultrasound beams (controlled by both transmit and receive processing) from the related M different vantage points. A problem with such methods is that in order to control the location of the vantage points, the transmit and receive apertures must be reduced since the aperture locations in part define the origin of the ultrasound beams and hence the vantage point. Instead of using the entire transducer element array, the transducer element section method described above uses portions of the transducer element array, which may significantly reduce the aperture size of the transducer array and the lateral resolution of the reflected images. In addition, the reduced aperture size of such methods may significantly reduce the signal strength and decreases the signal to noise ratio for the received target echoes.
Another known method for compounding an ultrasonic image is to perform a technique known as frequency compounding. An example of a frequency compounding technique is disclosed in U.S. Pat. No. 4,350,917, to Lizzi et al., entitled, xe2x80x9cFrequency-Controlled Scanning of Ultrasonic Beams.xe2x80x9d In accordance with Lizzi, a transducer having a piezoelectric element may be used with a transmit signal having a varying frequency to control the radiation direction of an ultrasound transmit beam. A problem with frequency compounding is that the axial resolution may be adversely affected.
The design of a system using spatial compounding usually involves engineering trade-offs and compromises. In a real time system, frame rates may be reduced because two or more images must be used to form each frame. In addition, larger apertures or several apertures must be used to provide different vantage points from which to create images. If the images are not created simultaneously, they must be buffered prior to being combined. As a result, additional resources, such as transducer apertures, related processing channels, and image buffer memory, can increase system cost and increase the size of both transducers and the ultrasound imaging systems.
All prior art techniques used to reduce ultrasound image speckle have the undesired consequence of reducing lateral and/or axial resolution of the target image. For example, lateral spatial compounding reduces lateral resolution of the image. By way of further example, frequency compounding reduces axial resolution of the image.
As a result, there is a need for a system and method that provides an improved two-dimensional image with reduced image speckle.
The present invention provides for an ultrasound imaging system configured to transmit and receive a plurality of ultrasound planes spatially separated and/or steered in elevation, to spatially compound in the elevation dimension for the purpose of creating an improved two-dimensional ultrasound image having reduced speckle.
Architecturally, the ultrasound imaging system may include a phased, linear, or curved linear array transducer in electrical communication with an ultrasound system controller configured to generate and forward a series of excitation signals to the transducer. The ultrasound imaging system may work in conjunction with the transducer to transmit ultrasound energy into a region of interest in a patient""s body along a plurality of transmit lines. A transmit scan beam may be defined by a plurality of transmit scan lines. The ultrasound imaging system, may further comprise a receiver for receiving ultrasound echoes with the transducer from the region of interest in response to the ultrasound energy and for generating received signals representative of the received ultrasound echoes. The system may also comprise a parallel beamformer for processing a plurality of received signals to form first and second sets of received ultrasonic beams which originate at first and second spatially separated vantage points, respectively. In accordance with the present invention, a plurality of received ultrasonic scan beams may be steered and focused at multiple points along the transmit scan beam to simultaneously generate first and second beamformer signals representative of ultrasound echoes received along each of the transmit lines.
The ultrasound imaging system is further configured to receive and recover information from ultrasound target echoes for further processing by any number of devices capable of translating the recovered ultrasound target echo information into a viewable image.
The present invention may also be broadly viewed as providing a method for ultrasound imaging. Briefly stated, the method comprises the following steps: transmitting ultrasound energy into a region of interest such that a transmit scan beam is formed; recovering a plurality of steered ultrasound response planes from ultrasonic echoes, each ultrasound response plane having a separately defined vantage point such that at least two ultrasound response planes are focused or steered in the elevation dimension to intersect at the transmit scan beam; deriving image information from the plurality of ultrasound response planes; and compounding the image information in the elevation direction.
In accordance with the method for ultrasound imaging, a plurality of response planes are steered and focused such that corresponding response planes intersect the transmit beam at a plurality of predetermined points along each of the transmit scan beams to simultaneously generate first and second beamformed signals representative of ultrasound echoes received at various distances from the transducer face along each of the transmit scan beams. The first and second beamformed signals are detected to form first and second detected signals, respectively. In the preferred embodiment of an ultrasound imaging system of the present invention, the first and second detected signals are mathematically combined to provide an image producing signal representative of an image of the region of interest.
In accordance with a first preferred embodiment of the method for ultrasound imaging, the response planes are received through first and second apertures, respectively, of a transducer array. The first and second apertures may be varied dynamically during reception of ultrasound echoes along each of the transmit lines.
In accordance with a second preferred embodiment of the method for ultrasound imaging, the first and second spatial vantage points may be maintained in fixed positions by defining fixed first and second apertures during reception of ultrasound echoes for each of the transmit lines.
In accordance with an alternative embodiment of the method for ultrasound imaging, a plurality of substantially parallel transmit scan beams closely separated in the elevation dimension may be applied to a region of interest. A plurality of spatially separated vantage points defined by respective apertures on the transducer array may be used to recover a plurality of response planes steered and focused to intersect at each of the respective transmit scan beams.
In accordance with another alternative embodiment of the method for ultrasound imaging, one or more of the aforementioned methods for spatially compounding in the elevation dimension is combined with one or more prior art methods for spatial compounding.
Other features and advantages of the invention will become apparent to one skilled in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the present invention.